Hepatocellular carcinoma (HCC) is responsible for more than 500,000 estimated deaths worldwide each year. The lack of effective therapies for this disease necessitates novel therapeutic approaches. Oncolytic viral vectors, such as those based on vesicular stomatitis virus (VSV), are a promising new anticancer platform for HCC. While the oncolytic effects of VSV are clearly defined, the mechanisms underlying this antitumor activity are incompletely understood. In addition to direct cytolysis, viral infection of cancer cels has been shown to elicit specific and nonspecific immune responses, thus overcoming immunological tolerance to the tumor. The type III IFN family of antiviral cytokines (IL-29, 28A, 28B; also known as IFN-?1, 2, and 3) have antiviral and immunomodulatory properties, and inhibit tumor progression in mouse models of cancer. We hypothesize that expression of these cytokines from VSV will improve oncolytic VSV activity by both immunostimulatory effects and through increased selectivity of the virus for tumor cells. Our preliminary data show that VSV expressing IL-28A is attenuated in cultured IL-28-sensitive immortalized hepatocytes and after intranasal delivery to mice in vivo. Using a mouse model of HCC, we will examine the hypothesis that expression of IL-28 from a VSV vector will augment oncolytic activity. We will then investigate the mechanisms by which IL-28 expressed from VSV enhances the oncolytic activity of VSV by performing mechanistic studies to determine the contribution of stimulatory effects on NK cells and T cells, increased selectivity of the virus for IL-28-resistant tumor cells and anti-proliferative/pro-apoptotic actions. Broadly, the studies proposed in this investigation will increase the understanding of antitumor mechanisms of oncolytic viruses for application as alternative therapies for HCC and other cancers. Our studies will also provide a better understanding of the complex and dynamic relationship between the host immune system, the tumor, and the virus, and provide insight into the regulatory mechanisms manipulated by cancer cells to evade elimination by the immune system.